Cell therapy seeks to provide biologically active molecules to a patient by implanting cells that produce the biologically active molecules into the patient. In unencapsulated cell therapy approaches, "naked" cells are implanted. Several approaches have been taken to prevent rejection of the implanted "naked" cells. Treatments include immunosuppression of the patient, preclearance from the recipient serum of natural antibodies, or administration of high doses of low molecular weight haptens to inhibit natural antibody binding to transplanted tissue. Alternatively, researchers have proposed the alteration of cells to reduce or eliminate expression of antigen or epitopes that stimulate rejection of the cells or tissue by natural antibodies in a recipient. However, using mitotically active cells creates a risk of tumorigenicity of the implanted tissue.
In encapsulated cell therapy approaches, a permselective physical barrier immunoisolates the implanted tissue from the host tissue. The barrier permits passage of the desired molecules between the patient and the encapsulated tissue, but protects the encapsulated cells or tissue from destruction by the immune system of the patient. Use of xenogeneic tissue or cells in encapsulated cell therapy also acts as a "safety feature," because encapsulated cells are rejected by the patient's immune system if the capsule breaks or ruptures.
A patient's immune system has several components, some of which are useful for encapsulated cell therapy and some of which are undesirable. In one component, phagocytes scavenge target cells, such as the xenogeneic cells described above. In particular, antibody-dependent cell-mediated cytotoxicity (ADCC) has an important role in the destruction of many target cells, including tumor cells, by macrophages. Opsonization of target cells with immunoglobulin G (IgG) enhances the removal of these materials from a host. The role of macrophages in the destruction of target cells by ADCC in the presence of specific antibodies has been well established. While the selectivity of macrophage targeting is based on antibody specificity, the lytic attack on the target cells is triggered by Fc receptor-mediated ADCC.
Another component of the immune system is the activation of the complement system. The two pathways of complement activation (the classical and the alternative pathways) are both directed at a central step in complement activation, the cleavage of C3. A single terminal pathway is the formation of a membrane attack complex (MAC). The classical pathway is normally activated by antigen-antibody complexes., where certain antibodies are complement fixing (capable of binding to complement to cause activation of the classical pathway). Activation of the classical pathway can be initiated with binding of C1q, the first factor of complement cascade, to the Fc region of immunoglobulin. Then, a cascade of proteolytic events results in the activation of C5 convertase, which cleaves C5 into C5b and C5a. The C5b then binds C6, C7, C8 to form a C5b-8 complex. Binding of C9 molecules to C5b-8 forms C5b-9 (the MAC), which inserts into lipid bilayers and forms transmembrane channels that permit bidirectional flow of ions and macromolecules. By this mechanism, complement causes lysis of the cells.
The complement system is important in host defense, but activation at inappropriate sites or to an excessive degree can cause host tissue damage. Complement is a factor in the causation or propagation of tissue injury in numerous diseases. For encapsulated cell therapy approaches in humans, therefore, (1) unencapsulated cells should be rejected immediately by the host; (2) encapsulated cells should be non-immunogenic to the host; and (3) the cell elimination process should not lead to immunological memory of the host. Accordingly, it would be desirable to be able to deliver a biologically active molecule to human patients using encapsulated cells or tissue that both have a human immunological "background," but also provide the safety feature of rejection by the patient in the event of capsule rupture or failure.